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More Genetics Complete Dominance • The pattern we’ve seen so far • Homozygous dominant and heterozygous are indistinguishable • YY and Yy will both make yellow seed Complete Dominance • Autosomal recessive pattern:: • AA, Aa are “normal” • aa has the condition • Autosomal dominant pattern: • AA, Aa have the condition • aa is “normal” Some patterns of inheritance don’t follow the rules. • Incomplete dominance • Heterozygotes have a blended appearance between both alleles • Ex. Blended colors in flowers • Codominance • Heterozygotes have the phenotype of both alleles expressed at the same time • Ex. Roan horses (next slide) What would you expect in each situation? INCOMPLETE DOMINANCE • Red flower x white flower = pink CODOMINANCE • Red flower x white flower = red and white flower mix Multiple Alleles • Instead of just two alleles (A vs. a), more than two alleles control a trait • Human blood types work this way • Alleles = IA, IB, i • IA & IB = Codominant Blood Typing Phenotype (Blood Group) Genotype(s) Type A IAIA or IAi Type B IBIB or IBi Type AB IAIB Type O ii Blood Typing Problem: • A man who is heterozygous for Type A blood marries a woman who is homozygous for Type B blood. What possible blood types might their children have? Blood Typing Problem 2: • A woman is IAIB. She has a child that is Type AB. Which genotypes might the father have had? Polygenic Inheritance: the effect of 2 or more genes acting upon a single phenotypic character (eg. skin color, height) These girls are fraternal twins! • http://www.parentdish.co.uk/2012/03/31/beautiful-black-and-white-twins-kian-andremee-hodgson-turn-seven/ Pleiotropy: one gene influences multiple, seemingly unrelated phenotypic traits Note the differences! • Multiple alleles • More than two alleles code for a trait • IA, IB, i • Polygenic inheritance • More than two genes influence a trait • Gene 1, gene 2, gene 3, gene 4 all influence skin color • Pleiotropy • One gene influences many phenotypes • The “Frizzle Gene” mutation makes chickens have curled feathers. This ALSO causes issues with body temperature, metabolism, blood flow, digestion. Lethal Alleles • First, cross Ll x Ll • Some genotypes are lethal and cause death (generally, before an organism is born) • LEAVE OUT LETHALS when figuring genotype, phenotype, and probability. DEAD IS DEAD! You have no phenotype if you are dead. • The ratios that would result if ll was lethal: Practice • For a certain disease, HH is a lethal allele that causes death before an organism is born. If two parents, Hh x Hh, have kids, what is the chance that they will have a child that is homozygous recessive? Some traits are non-nuclear • Genes on chloroplasts and mitochondrial DNA do not follow Mendelian rules • During meiosis, chloroplasts and mitochondria are randomly assorted to gametes. • Also, since mitochondria are passed via the egg only, certain traits come only from your mother Nature and nurture • Both genetic and environmental factors influence phenotype • Example: hydrangeas change color/intensity based on the pH and aluminum content of the soil Pedigrees- diagrams to show the relationship between parents/offspring across 2+ generations. Identify each pedigree as autosomal dominant inheritance, autosomal recessive, or neither. Identify each pedigree as autosomal dominant inheritance, autosomal recessive, or neither. Identify each pedigree as autosomal dominant inheritance, autosomal recessive, or neither. Identify each pedigree as autosomal dominant inheritance, autosomal recessive, or neither. Genetic Testing • Most children with recessive disorders are born to parents with normal phenotypes, so determining if parents are heterozygous is useful. • Many tests are around that can screen for carriers of disorders like Tay-Sachs, sickle-cell, CF, and others. • Consider health insurance. What sorts of ethical issues might knowing your phenotype (especially if a disease will manifest later) bring up? • What about employers—if they don’t understand what “carrier” means and they get a hold of your data, what might that lead to? • Tests are out there for Huntington’s. Would you want to know? Genetic Testing • You will read more about genetic testing and ethical issues in your book. 279-281 Autosomal Recessive Disorder: Cystic Fibrosis • Most common lethal genetic disease in the U.S. • Normal allele: codes for membrane protein that transport chloride ions • Mutant allele: codes for defective (or no) membrane protein • Pleiotropic effects: poor absorption of nutrients from intestines, chronic bronchitis, recurrent bacterial infections Autosomal Recessive Disorder: Sickle Cell Disease • Most common inherited disorder among those of African descent • Point mutation in hemoglobin gene • When oxygen is low, the sickle cell hemoglobin molecules change shape, which may clump and clog blood vessels Autosomal Recessive Disorder: Sickle Cell Disease • Phenotype varies due to genotype. • ss = sickle cell disease = full sickle cell disease • Ss = sickle cell trait = usually healthy, but may have some sickle-cell symptoms during prolonged periods of reduced blood oxygen • SS = normal = no sickle cell at all • IMPORTANT! Sickle cells cannot be infected by malaria. • So…there is an advantage to being Ss…if you have sickle cell trait, you can’t get malaria and you are also mostly normal. Autosomal Dominant Disorder: Huntington’s Disease • Is a lethal dominant…but the “lethal” part kicks in after reproductive age • Degenerates nervous system • No phenotypic effects until age 35-45; irreversible and fatal Autosomal Dominant Disorder: Achondroplasia • Homozygous dominant is lethal • Heterozygous individuals will have the dwarf phenotype • Homozygous recessive individuals (99.99% of the population!) are not dwarfs • How can a family with dwarf parents have a non-dwarf child?